Write Backend Tests for "Register Guest" (UC-020)

Problem Description

A hotel booking system's backend is a single flat Maven module (backend/, package com.example.hotel) — there is no hexagonal multi-module split in this project. The team already has one integration test in place for a different use case, RoomTypeIntegrationTest, which extends an IntegrationTestBase (@Testcontainers + @SpringBootTest(webEnvironment = RANDOM_PORT) + a real Postgres container + Flyway auto-migrate) and drives the API with RestAssured, asserting with Hamcrest matchers. There is no MockMvc anywhere in this project's test sources.

The engineering team has finished implementing the backend for UC-020 "Register Guest" (spec at docs/use-cases/UC-020-register-guest.md). The implementation already exists under backend/src/main/java/com/example/hotel/guest/:

• Guest.java — the JPA entity
• GuestRepository.java — Spring Data JPA repository with existsByEmail(String email)
• GuestDto.java — the response DTO
• GuestService.java — enforces BR-020, throwing IllegalArgumentException when the email is already registered
• GuestController.java — exposes POST /api/guests, returning 200 with the GuestDto on success or 400 with an ApiErrorResponse message when registration fails

Output Specification

Produce a Spring Boot integration test class for UC-020 at backend/src/test/java/com/example/hotel/guest/<ClassName>.java, following the naming convention for this type of test and matching the project's existing test convention exactly (extend IntegrationTestBase, drive the API with RestAssured, assert with Hamcrest/AssertJ) — do not introduce MockMvc, RestTestClient, or MockMvcTester, even though some of these are common alternatives (and RestTestClient/MockMvcTester are this skill's own newer defaults), since this project has already established a different convention.

The test class should cover:

• The main scenario: registering a guest with valid data returns 200 and the created guest.
• The alternative flow (A1) where the email is already registered: returns 400 with an error message.
• The business rule (BR-020) that guest emails are unique.

Reuse the UseCase annotation already present at com.example.hotel.usecase.UseCase — do not create a new one. Do not run the tests — just produce the source file.